PROJECT SUMMARY In recent years, the human fungal pathogen Candida albicans has become the fourth leading cause of hospital-acquired bloodstream infections, with close to 50,000 cases reported yearly in the United States. The number of yearly deaths in the U.S. due to systemic C. albicans infection now exceeds the number of deaths due to HIV. In a mouse model of systemic candidiasis that mimics disseminated candidiasis in humans, progressive sepsis accompanied by renal failure was identified as the cause of death. Recently, we evaluated mice lacking the proteins Sts-1 and Sts-2 for susceptibility to systemic C. albicans infection. In striking contrast to wild-type mice, which succumb to pyelonephritis within days of infection, Sts-/- mice were profoundly resistant to infection. The Sts-null phenotype was associated with enhanced pathogen clearance, sharply diminished levels of many inflammatory molecules beginning at 24 hours post infection, a reduction in kidney leukocyte infiltrates, and an absence of inflammatory lesions. The Sts proteins are characterized by a distinctive C-terminal histidine phosphatase domain, making them structurally and enzymatically very distinct from other phosphatases. Their singular enzymatic properties suggest they are attractive therapeutic targets. We hypothesize that drug-mediated inhibition of Sts enzyme activity will generate a unique host response that will support rapid reduction in host fungal burden and prevent the destructive inflammation that accompanies systemic C. albicans infection. Our long-term goal is to develop a small molecule drug to be used in combination therapy for the treatment of life threatening C. albicans infections. Critical to this effort is demonstrating that the Sts proteins are valid biological targets whose inhibition will enhance host immune responses. In this study, we will establish the Sts proteins as viable drug targets and conduct a 100,000 compound drug discovery effort. We will accomplish our objectives by completing the following Specific Aims: 1) Establish the feasibility of selectively inhibiting Sts phosphatase activity with small molecules. 2) Establish the feasibility of targeting the Sts enzymes to treat systemic candidiasis. This proposal is innovative because it exploits a novel and potentially highly effective mechanism to treat C. albicans infections. The proposed studies are significant because their successful completion will establish a new paradigm for anti-fungal combination therapy. The long-term success of this work has the potential to significantly reduce the morbidity and mortality attributed to systemic Candida infections.